1. Technical Field of the Invention
This invention relates to the control and reduction of fluid pressure in valves for controlling fluid flow, especially but not exclusively severe service valves for use in power industries and oil and gas wells.
2. Description of the Related Art
The technology most widely used in severe service valves utilizes pressure letdown chambers consisting of one or more flow passages containing multiple orifice openings, labyrinths, or multiple, abrupt angular turn, passageways resulting in a staged pressure reduction. Alternatively, flow restrictions can be afforded by providing in the flow passage a region of reduced cross sectional area through which the fluid passes. As the fluid flows through the flow passage, the velocity is locally increased in that region thereby generating downstream turbulence which dissipates energy and reduces the pressure.
In the afore-mentioned control valves, dissipation of the energy of the flowing fluid is effected by frictional drag through tortuous passages or by successive abrupt contractions and expansions through smooth passages. Both of these types of valve work well for clean fluids but in a number of applications the fluid will contain contaminants, for example solid particles or droplets of liquid (hereinafter collectively referred to as particles), which will cause the passages to erode quickly. This is especially prevalent in the tortuous passage type which rely on a series of 90 degree turns at which particles will collide with the passage walls, thereby accelerating erosion, and in expansion / contraction valves as the contraction has an associated acceleration of both the flow and the particles entrained therein, the speed with which the particles collide with the passage walls also increases thereby accelerating erosion. The problem of erosion is particularly prevalent in well head choke valves, which are the primary shut-off/control valve on the top of an oil or gas well head. Separators for separating at least a portion of the particles are sometimes used, however with choke vales, as they are immediately atop the well head, if a means of separating particles is placed upstream of this valve then they are hard to service and result in overly complex systems.
The present invention attempts to mitigate problems with existing valves by providing a control valve with increased erosion resistance.